In future cellular wireless access systems, as well as in some contemporary systems, a principle which may be used is the so called TDD principle, Time Division Duplex. In TDD systems, transmissions from the base stations to the user terminals in the cells, the “down link”, as well as transmissions from the user terminals to the base stations, the “up link”, are carried out on the same frequency, but with a division in time.
Due to the principle employed, in a TDD system there may be interference between the uplink and the downlink, as they are located on the same frequency. Thus, for example, a user terminal transmitting uplink to a base station may cause interference in another user terminal in an adjacent cell which is receiving in the downlink. Similarly, a base station transmitting in the downlink to a user terminal may cause interference in another base station which is receiving traffic in the uplink. As is well known, interference from another base station may be very high since base stations especially when there is line-of-sight between the base stations. Similarly, two terminals may be very close to each other, in which case very high levels of interference are also possible.
This problem, i.e. interference between uplink and downlink, can be solved by synchronizing and coordinating all base stations within a certain area, so that all uplink and downlink periods occur simultaneously.
As an additional safe guard against such interference, so called “guard periods” may be inserted at the transitions between the down link and the up link periods. Usually, there is a guard time of a first duration used at the transition from down link to up link, and a guard time of a second duration at the transition from up link to down link.
Typically, the guard time at the transition from downlink to uplink is chosen to match the sum of the maximum roundtrip propagation delay in a cell and the time it takes for a user terminal to switch from reception to transmission.
Thus, due to the propagation delay, there is a delay before a user terminal can receive the downlink data. In addition, the transmission timing is controlled so that terminals with a longer propagation delay, e.g. those at the cell edge, may start their transmissions earlier to compensate for the propagation delay, in order for the data to be received within the uplink window at the base station of the cell.
The guard period at the transition between the uplink and downlink, on the other hand, is typically chosen to match the time it takes for the base station to switch from reception to transmission and the time it takes for a (close) terminal to switch from transmission to reception.
However, despite the fact that all cells in a certain area, as mentioned above, may be synchronized, there may still be interference during the uplink period. At the end of the uplink period, a base station may, due to, for example, a synchronization error, start to transmit too early, i.e. it may “cross over” to the down link too early. These transmissions will then be received in other base stations which are still in their up link period, and are thus open for reception.
Also, down link “contributions” from distant base stations which are still “in the air” due to propagation delays in the beginning of the uplink period may cause interference at a base station.
In the case of base station to base station interference, this interference may, to some extent, be mitigated by channel coding, as only a part of the uplink period is affected by interference.
If the guard periods are chosen based solely on the propagation delays to the user terminals in the cells, there may still be base station to base station interference present in the system, as mentioned above. This may occur if the guard periods are too short, and do not take into account “contributions” from distant base stations as well as “contributions” from close base stations with synchronization errors.
It can be shown that interference contributions from a single distant base station with an elevated antenna may be heard at distances up to around 60-80 km, corresponding to propagation delays of around 250 us.
Additionally, there may be interference at one base station from multiple other base stations.
In principle, interference at the edge of the up link period may be handled with channel coding, but it may be noted that this will affect the up link coverage, and that the radio base station, including the RF front end, may need to handle a very high noise “rise” at the beginning and at the end of the up link period, and that the base station will need to account for this during demodulation. Thus, for small guard periods, a more sophisticated RF front end, in addition to more advanced BB demodulation processing is needed, which might increase the complexity in the implementation of the radio base station.
Thus, two drawbacks with shortened guard periods is that the up link performance, which is already limited by the limited user terminal transmit power, will be degraded, and that the complexity of the radio base station might increase, as compared to the case where these variations are not present.
In addition, if the guard periods are chosen to also take into account interference from other radio base stations, it may be noted that the required guard time will depend on the propagation conditions between the base stations, and will thus be “deployment dependent”. Additionally, as the system grows, new interference sources will be added.
Thus, the required guard time for inter-base station interference is expected to depend on the network layout, which is generally hard to predict.
Document EP 1 511 190 discloses a method by means of which an RBS can adapt the guard period between down link and up link transmissions in the cell of that RBS. This disclosure is, however, not directed at solving the problem of RBS to RBS interference.